UPPER WINDS – Jet streams and Rossby waves
Higher in the atmosphere there is less friction from the Earth’s surface that at the ground
level. As a result, the winds are stronger and are deflected more by the Coriolis force.
The outcome is that air circulation is more directly east-west than nearer to ground level.
In the tropics, the main movement is from east to west but, in higher latitudes, there is
much stronger air flows from west to east. Within these flows there are zones of
exceptionally high winds called JET STREAMS. Jet streams are fast flowing, relatively
narrow air currents found at the tropopause, the transition between the troposphere
(where temperature decreases with height) and the stratosphere (where temperature
increases with height), and are located at 10-15 kilometers above the surface of the Earth.
They form near boundaries of adjacent air masses with significant differences in
temperature, such as the polar region and the warmer air to the south. The path of the jet
typically has a meandering shape, and these meanders are one manifestation of ROSSBY
WAVES. Jet sterms can be used to explain why a flight from N.America to Europe is
shorter than the return trip.
The winds in the upper troposhere do not flow in parallel line directly around the globe.
Those at higher latitudes have a wave-like pattern called ROSSBY WAVES.
Jet streams flow from the west in the upper portion of the troposphere. Diagram below
shows the meandering Rossby Waves.
The movement of the Rossby waves and the polar front jet stream affects the weather in
temperate latitudes. Alternating settled conditions which are associated with high
pressure, and changeable conditions associated with low pressure, are due to complex
changes in the upper troposphere. The exact causes of Rossby wave patterns are not well
understood but features such as mountain ranges and oceans are thought to play a part.
The waves are also regarded as unpredictable.